Fire sprinkler systems are a well-known type of active fire suppression system. Sprinklers are installed in all types of buildings, commercial and residential, and are generally required by fire and building codes for buildings open to the public. Typical sprinkler systems comprise a network of pipes, usually located at ceiling level, that are connected to a reliable water source. Sprinkler heads are disposed along the pipes at regular intervals. Each sprinkler head includes a fusible element, or a frangible glass bulb, that is heat-sensitive and designed to fail at a predetermined temperature. Failure of the fusible element or glass bulb opens an orifice, allowing water to flow through the head, where it is directed by a deflector into a predetermined spray pattern. Sprinkler systems may suppress a fire, or inhibit its growth, thereby saving lives and limiting inventory loss and structural damage. Sprinkler specifications are published by the National Fire Protection Association (e.g., NFPA 13, 13D, 13R).
The sprinkler system (more generally, Fire Protection System, or FPS) is fed from a pump room or riser room. In a large building the FPS consist of several “zones,” each being fed from a riser in the pump room. The riser contains the main isolation valve and other monitoring equipment (e.g., flow switches, alarm sensors, and the like). The riser is typically a 6 or 8 inch diameter pipe coupled through a booster pump (called the fire pump) to the main water supply to the building. The riser then progressively branches off into smaller “cross mains” and branch lines, also known as “zones”. At the furthest point from the riser, typically at the end of each zone, there is an “inspector's test port,” which is used for flow testing.
Many FPS are “wet” systems—meaning the sprinkler pipes in each room are full of water under a predetermined “internal set point” pressure. If the water pressure decreases below the set point, valves are opened and/or a pump is activated, and water flows into the sprinkler pipes in an attempt to maintain the pressure. The set point pressure drops when water escapes the system, such as due to the opening of a sprinkler head in a fire. However, the system may also be activated by a broken sprinkler head, or leaks in the system, such as leaks caused by corrosion of the pipes.
Due to the possibility of water discharge in other than actual fire conditions, a wet FPS present an unacceptable risk to sensitive equipment or merchandise in many applications. For example, a data center that houses expensive, mission-critical computing or telecommunications equipment; a semiconductor manufacturing facility; and a warehouse storing high-value, non-waterproof merchandise, are examples of facilities in which a wet FPS would be unacceptable. Also, areas subject to freezing temperatures cannot utilize wet FPS.
To address the need for FPS in areas where a wet FPS is not acceptable, alternatives to the wet FPS have been developed. These are of two general categories. Dry FPS are typically used in areas that are subject to freezing temperatures, where a water-filled system is not practical (e.g., parking garages, non-heated attics of motels and nursing homes, and the like). A dry FPS uses compressed air in the piping as a “supervisory gas.” The air is maintained at a supervisory pressure, e.g., approximately 20 PSI. When a sprinkler head opens, the air pressure drops to atmospheric (e.g., 0 PSI), and a valve opens in response to the lower pressure. The valve locks in the open position and water rushes into the system. Dry FPS address the freezing problem, but present the same hazards of loss or damage to expensive equipment or merchandise as wet FPS, if the dry FPS is activated due to sprinkler head damage or failure, or a leak such as from corroded pipes.
Pre-Action FPS, also called a double interlock dry FPS, protects against water damage by increasing the probability that the system is only activated by an actual fire. A pre-action FPS operates similarly to a dry FPS; however, two or more action signals are required before water is injected into the system. A drop in supervisory air pressure alone will not activate the water isolation valve unless a second signal, such as a heat or smoke detector signal, is received by the control panel. At that point the isolation valve will open and water will rush into the zones with the aid of a booster pump called the fire pump.
Both dry and pre-action FPS must be hydro-tested after initial installation to make sure that the piping and hangers can support the additional weight of the water, and to make sure that the flow rate of water through the system conforms to applicable specifications (e.g., the NFPA 13 standard). Once it has passed all the tests, the system is drained and then filled with compressed air (supervisory gas). However, the FPS pipes never drain completely, and the residual water that remains creates ideal conditions for the initiation and propagation of corrosion in the piping either by means of galvanic or organic induced corrosion. Sometimes, microbes can grow in the water and accelerate the corrosion by means of the byproducts that they produce during their metabolic cycle. This is called Microbiologically Influenced Corrosion (MIC). Over time, MIC or galvanic corrosion can cause extensive damage to an FPS, as corrosion-induced leaks cause loss of supervisory gas, either arming (pre-action FPS) or activating (dry FPS) the system. Compressed air is used to maintain the supervisory pressure in both Dry and Pre-Action FPS, and this compressed air provides the oxygen that induces the galvanic corrosion and/or MIC, when the FPS is laden with residual water after draining from the hydrotesting.
Most real-world dry and pre-action FPS leak pressure. The allowable leak rate 1 PSI per 24 hours, per NFPA specifications. Conformance with this leak rate is tested following the hydro-testing, and if it is in compliance, the FPS is certified. From that point on, the supervisory gas leak rate only increases over time, due to dried out pipe coupler gaskets, fitting compounds (i.e., “pipe dope”) drying out, and the like. In practice, most building owners are not concerned about the increased in leak rate until the air compressor providing the supervisory gas begins to cycle excessively to maintain the supervisory pressure. Eventually, it cannot maintain the required pressure, and the FPS will trigger an alarm, requiring that the leaks be found and sealed, or that a larger compressor be installed.